Microanalyser convertible into a mass spectrometer

ABSTRACT

A microanalyser operating by secondary ion emission and comprising a double magnetic-prism for deflecting the ions according to their &#39;&#39;&#39;&#39;momentum-to-charge&#39;&#39;&#39;&#39; ratio and electrostatic means for filtering the ions according to their &#39;&#39;&#39;&#39;energy-tocharge&#39;&#39;&#39;&#39; ratio. An element is provided to operate either as an electrostatic mirror allowing the production of images through ion microscopy or as a transmitting and filtering device incorporated in the make-up of a double-focussing mass spectrometer in accordance with the magnetic prism and an electrostatic condenser.

United States Patent Vastel MICROANALYSER CONVERTIBLE INTO A MASS SPECTROMETER Inventor: Jean Vastel, Courbevoie, France Assignee: Compagnie DApplications Mecaniques a LElectronique au Cinema et a LAtomistique (C.A.M.E.C.A.), Paris, France Filed: July 17, 1973 Appl. No.: 379,925

Foreign Application Priority Data July 21 1972 France 72.26420 U.S. Cl. 250/296, 250/309 Int. Cl Hlllj 39/34, HOlj 37/26 Field of Search 250/309, 396, 397. 306

References Cited UNITED STATES PATENTS Castaing 250/309 Feb. 11, 1975 3,686.499 8/l972 Omura 250/309 Primary Examiner-.lames W. Lawrence Assistant Examiner-C. E. Church Attorney, Agent, or FirmCushman, Darby & Cushman {57] ABSTRACT A microanalyser operating by secondary ion emission and comprising a double magnetic-prism for deflecting the ions according to their momentum-to-charge" ratio and electrostatic means for filtering the ions according to their energy-to-charge" ratio. An element is provided to operate either as an electrostatic mirror allowing the production of images through ion microscopy or as a transmitting and filtering device incorporated in the make-up of a double-focussing mass spectrometer in accordance with the magnetic prism and an electrostatic condenser.

2 Claims, 3 Drawing Figures PATENTEU 3, 866.042

SHEEI P u 2' F I G 2 1s WE III/III I VII/III H l HIGH- VOLTAGE SOURCE PIC-3.3 60 sa MICROANALYSER CONVERTIBLE INTO A MASS SPECTROMETER The present invention relates to a microanalyser operating by secondary ion emission. Microanalysers of this kind utilise secondary ion emission in order, through a system combining ion microscopy and mass spectrometry. to produce images which are characteristic of the surface of a sample, and show the distribution, on said surface, of the constituent chemical elements which go to make it up.

Known microanalysers of this kind, such as that described in [1.5. Pat. No. 3,500,042 (Bristish Patent Specification No. l,l28.85l operate on a double-filter principle. achieved by means of a magnetic prism and an electrostatic mirror, which are associated with one another. Also. the filtered ions are employed, with the help of a measuring device, to determine the relative amounts of the various kinds of ions, and to produce mass spectra pertaining to restricted zones of the sample. As the microanalyser when used as a mass spectrometer operates with only the lower energy ions which are reflected by the electrostatic mirror it is difficult to distinguish therewith between the atomic and the combination ions. As the atomic ions show an energy distribution which is generally broader toward the high energies than that of the combination ions, it is of interest to carry out measurement on the higher energy atomic ions by means of a microanalyser combined with an additional arrangement according to the invention.

It is an object of the invention, by the addition of a restricted number of elements, to produce a microanalyser operating by secondary ion emission, which can operate as a mass spectrometer of high resolving power, whilst retaining the optical properties of a system for transmitting the ion image, and which enables rapid conversion from one mode of operation to the other.

According to the invention there is provided a microanalyser convertible into a double-focussing mass spectromet comprising: a source for emitting a secondary ion beam; 21 first magnetic prism for deflecting said ion beam; a convertible element located on the trajec tory of the ion beam deflected by said first prism, said element selectively operating as an electrostatic mirror for reflecting said ion beam reduced to a predetermined energy range or as a component for filtering said ion beam and letting it traverse therethrough', means for controlling said convertible element; a second magnetic prism for deflecting the reduced ion beam reflected by said element; an image converter for receiving said reduced ion beam deflected by said second prism; an electrostatic deflector for deflecting said filtered ion beam traversing said element; and ion collecting means for receiving said filtered ion beam deflected by said electrostatic deflector.

The invention will be better understood and other of its features rendered apparent, from a consideration of the ensuing description and the accompanying related drawings in which:

FIG. 1 is a simplified diagram, in the plane of the optical axis, of an embodiment of a microanalyser in accordance with the invention;

FIG. 2 is a view, partially schematic, of elements of the electrostatic mirror in the microanalyser shown in FIG. 1;

FIG. 3 is a sectional view of a microanalyser in accordance with the invention limited to the part constituted by the electrostatic mirror and the input of the electrostatic deflector.

The microanalyser of FIG. 1 comprises a primary ion source which bombards the sample 2 being studied, an electrostatic lens 3 for accelerating the secondary ions which are emitted by the sample and for focussing the ion beam formed, a magnetic deflector system 4, an electrostatic mirror 5, an ion-electron image-converter device 7, and, preceding the latter, a second lens 6.

The magnetic deflector 4 is constituted by two prisms. These prisms are delimited by two angles. respectively BAX and ABy in the plane ofthe optical axis. each angle being around 45 and the prisms having a common face AB. The two prisms are symmetrical in relation to the plane 0 bisecting AB. The variable magnetic field is normal to the plane ofthe figure and generated by poles whose faces take the form of the element shown in full line at 4.

The common axis of the lens 3 and the lens 6 is parallel to the straight line AB, at a distance R therefrom. The axis of the mirror 5 is the bisector of AB.

The optical axis of the equipment (mean trajectory of the beam) is symmetrical in relation to the plane Q. It is constituted, over its first half, by the axis of the lens 3 up to the first prism. then within the first prism, by a quarter of a circle of radius R tangentially joining the axis of the mirror, and, finally, at the exit from the first prism, by said latter axis itself.

The axes of the lenses 3 and 6 include the object axis of the first prism and the image axis of the second prism. The axis of the mirror is the image axis of the first prism and the object axis of the second prism.

Each of these two prisms comprises, in respect of the optical axis considered. a pair of real, conjugate. stigmatic points C, (object point) and C (image point), in the case of the first prism, and C (object point) and C, (image point) in the case of the second prism.

Each prism. furthermore, comprises a pair of virtual, conjugate, stigmatic points within the magnetic deflector, namely D, (object point) and E (image point) in the case of the first prism, and E (object point) and D, (image point) in the case of the second prism.

in a preferred embodiment of the microanalyser which secures optimum conditions both of stigmatism and of reduction in chromatic" aberrations (aberrations due to differences in velocity of the ions concerned), all the following conditions are combined:

The cross-over of the beam coming from the lens 3 is at C, and the lens is adjusted in order to produce an image of the sample in the neighbourhood of D,', the mirror 5 is the equivalent of a convex mirror whose centre is at C and its apex at O; a diaphragm is located at C, which may be constituted by the first electrode of the mirror; an element for correcting residual astigmatism (not shown), is placed in the neighbourhood of C The mode of operation involved in producing an image is as follows: the field in the double prism is adjusted so that the ions P, characterized by their massto-charge" ratio m,,/q and their initial velocity v,,, follow the optical axis if their initial velocity is directed along said axis. The diaphragm centred at C carried out filtering as a function of the momentum-to-charge ratio and the reflective power of the mirror is adjusted so that it only reflects towards the double prism ions whose energy-to-charge ratio is less than a given threshold, the

others falling onto the final electrode of the mirror. The diaphragm aperture and the reflective power of the mirror are adjusted so that only ions of mass m,, are reflected towards the double prism. After a second transit of the double prism. these reflected ions are utilised to form the final ion-image by means of the lens 6, this image being transformed into an electronic image by means of the converter 7.

In accordance with the prior art. the electron beam emitted by the target of the ion-electron converter under the impact of the final ion beam is utilised for purposes of mass spectrometry (this electron beam then being deflected away from the axis of incidence of the ions).

In accordance with the invention, the portion of the equipment described up to the mirror (in the direction followed by the ions), is utilised in association with a condenser 8 playing the part of an electrostatic deflector, a diaphragm ll of variable aperture and an ion collecting device 9, to constitute a double-focussing mass spectrometer with an intermediate convergence point. for example of the Nier Johnson type, the magnetic prism then being located before the electrostatic deflector. These mass spectrometers have been described more particularly in volume NO. 91 of Physical Review. published in July I953. A switching device makes it possible, on the one hand, to render the mirror electrically inoperative and, on the other hand, to open passage towards the electrostatic deflector, for the ion beam issuing from the first prism.

In mass spectrometers the main concern is with fo cussing in the radial plane (this plane being defined as the plane containing the optical axis and being perpendicular to the magnetic field), the ions being separated by means of slots located perpendicularly to the said plane. In double-focussing spectrometers, there are carried out simultaneously for ions of given mass-tocharge ratio, directional focussing (within a small angle) and focussing within a velocity range, this latter focussing being achieved by compensation between the chromatic aberrations due respectively to the magnetic prism and the electrostatic deflector.

In the preferred embodiment of the present invention, the electrostatic deflector is a condenser of spherical type, thus chosen in order to have small dimensions and to achieve directional focussing not only in the radial plane but also in the transverse section (surface perpendicular to the radial plane); the spherical condenser has the advantage of producing in the ions of mass m,,, a relative dispersion in accordance with their energies, which is double that produced by a cylindrical condenser of the same radius, so that for a given dispersion it is possible to utilise a spherical condenser whose radius is only half the radius of the corresponding cylindrical condenser. The property of focussing in the transverse section, although not essential, is nevertheless advantageous from the point of view of the overall sensitivity of the mass spectrometer.

This condenser 8 is a 90 deflector constituted by two electrodes. 81 and 82, having spherical surfaces concentric with one another; the value of 90 chosen for the angle of the deflector. facilitates the technological realisation of the condenser and furthermore leads to a reduction in bulk because of the inherent properties; the condenser has an optical axis constituted, by a circular arc of 90 concentric to the electrodes and having a radius which is the mean of the radii of the electrodes,

and by straight sections which extend said circular are beyond the entry and exit faces of the condenser, which straight sections, with their respective projections, constitute the object axis and the image axis of the condenser.

The spherical condenser is a stigmatic system producing in respect of each object point on the object axis, a conjugate image point on the image axis. The object axis of the condenser is coincidental with the image axis of the first magnetic prism and on the other hand the optical axes of the first prism and the condenser, constituting the optical axis of the spectrometer, are located at one and the same side in relation to said common axis.

The diaphragm ll constituting the input of the ion collector, is centred on the image point which is the counterpart of the point C relatively to the spherical condenser.

The distance of the image axis of the condenser from the object axis of the first prism is determined in order to achieve cancellation of first order chromatic aberrations.

The mean radius of the condenser is designed, on the one hand, in order to achieve at least partial compensation of the second order angular aberrations, and on the other hand in order to leave the necessary space for the mirror electrodes in front of the input face of the condenser, that is to say in order not to substantially modify the initial structure of the microanalyser.

The electrostatic mirror 5 is represented in the diagram of FIG. 2 and in FIG. 3, where common references are used to indicate the same elements. The mirror has symmetry of revolution about an axis and comprises three electrodes 52, 53, 54 respectively connected to three outputs of an electric supply device, 12, one of said outputs being an earthing point. These electrodes are mechanically mounted by insulating components. The two first electrodes 52 and 53 have an annular structure and the first electrode 52 is supplemented by a diaphragm 51 having the form of a variable-width rectangular slot, designed to filter the ions coming from the first magnetic prism.

The aperture of this diaphragm is controlled by two identical devices; each of these devices comprises a control knob such as that marked 17, screwed onto a threaded body; these knobs, through the medium of levers, make it possible to move apart the displaceable edges of the diaphragm, by overcoming the action of the springs such as that marked 18.

The final electrode 54, which constitutes the base of the mirror, here forms part of a plate 50 comprising another part containing an opening; the plate 50 can slide on an electrode mounting 55 so that it can occupy two distinct positions: a first position, that shown in FIGS. 2 and 3, in which the opening is offset from the optical axis in order not to interfere with the lines of force of the electric field between the electrodes of the mirror; and a second position in which the opening is centred on the optical axis in order to let a beam through.

A single control knob 59, screwing onto a threaded body 60, makes it possible, through the medium of a control thrust pin 58 and an insulating thrust pin 57, to displace the moving plate in order to place it in the second position against the action of a spring 61 which seats on the plate through a second insulating thrust pin 56.

This said single control knob simultaneously controls the displacements of the moving plate and the switching of the high-voltage supply to the electrodes; during its travel. the knob 59 triggers a mechanical contactbreaker 16 connected in series with a supply source in the circuit of the energising coil of a relay l5; this relay, in turn, controls high-voltage switches (marked symbolically by the reversing switches 13 and 14 in FIG. 2) arranged in the supply circuits of the electrodes 53 and 54.

The electrode mounting SS is made ofa material having good thermal conductivity; it has a concave portion darkened with black nickel in order to absorb the radiated energy emitted, through a transparent sealed dome 63. by a heating lamp 62. This device makes it possible to reduce the contamination of the electrode 54, the latter being heated by thermal conduction and raised to a higher temperature than the neighbouring components.

In the mass spectrometer mode of operation, the electrodes of the mirror are grounded, and the plate 50 is in its second position. The ions coming from the first magnetic prism have a trajectory close to that indicated by an arrow F in FIGS. 2 and 3; after having been filtered by the diaphragm 51, they pass through the free space zone. which is electrically neutral and is loated inside the mirror, and enter the electrostatic deflector through the opening in an entry plate 83, such entry plate being located in proximity of the electrodes 81 and 82 of the spherical condenser.

The device constituted by the first part of the known analyser, up to and including the diaphragm 51, the condenser 8, the diaphragm 11 and the collector 9. constitutes the double-focussing mass spectrometer.

The angular focussing property in the plane of the optical axis and in the transverse section normal to said plane. and the property of velocity focussing, make it possible to achieve a high resolving power whilst reducing the sensitivity of the equipment to a lesser degree than is the case with the prism and mirror system. By reducing the aperture of the diaphragm 51 filtering is improved, but, as far as the desired ions of mass m are concerned, the range of velocities around the velocity v that is to say the energy band of the useful ions, is reduced. By reducing the aperture of the diaphragm 11, the mass resolving power of the equipment is increased. The filtering effected by this diaphragm is improved as a punctual image is provided and not the image of a slot as in conventional spectrometry.

Taking account ofthe respective geometric positions of the various elements, the ion beam enters the first prism at an angle of incidence in relation to the normal to the entry face. If, as before R designates the radius of curvature of the optical axis within the magnetic deflector, then the microanalyser has the following dimensions:

distance between the point C, and the first intersection between the optical axis and the first prism 2 R;

distance between the point C and the second intersection between the optical axis and the first prism (/2 lg R.

The double-focussing property is thus obtained for a distance of (3/2 tg R/2 between the image axis of the condenser and the point C.

If R is the radius of curvature of the optical axis inside the condenser, then there is the following distance between the diaphragm l1 and the exit face of the condenser:

For a radius R close to 0.7 R, partial compensation of the second order angular aberrations is achieved, along with a reduced bulk on the part of the system constituted by the condenser and the ion collector 9.

The embodiment of the invention is capable of modification. ln particular. it is not essential to utilise a spherical condenser. in a general way, any condenser could be used which is capable. together with the first prism of the analyser. of forming a double-focussing spectrometer.

Of course, the invention is not limited to the embodiments described and shown which were given solely by way of example.

What is claimed is:

1. Means for selectively forming an ion microanalyset or a mass-sectrometer of the double-focusing type, said means comprising:

a sliding plate having a solid first part and a second part provided with an aperture;

an ion microanalyser comprising: first means for deriving secondary electrons from a sample and focusing those secondary electrons into a beam having a cross-over; first and second magnetic prisms having a common face and respective second faces; means for directing said beam toward said second face of said first magnetic prism; an electrostic reflecting means, provided with an entrance diaphragm centered at the image point of said crossover relatively to said first magnetic prism, for reflecting toward said common face those of said secondary ions, let through by said diaphragm, whose energy-to-charge ratio lies under a predetermined threshold, said reflector means comprising a plurality of electrodes including a solid last electrode formed by said solid first part of said sliding plate; an ion electron converter; and means for directing the ions issuing from said second face of said second magnetic prism toward said ion converter;

a mass-spectrometer comprising, in series: said first means; said first magnetic prism; said diaphragm; and energy filtering electrostatic deflector means; and an ion collector;

means for imparting to said sliding plate a first position for which said first part of said plate forms said last solid electrode of said reflector means, and a second position for which said aperture of said second part is coaxial with said diaphragm; and means for switching the potentials applied to at least two of said electrodes of said reflector means according to whether said plate is in its first or second position.

2. An apparatus as claimed in claim I, wherein said electrostatic deflector means is a spherical electrostatic deflector means. and wherein said ion collector comprises an entrance-diaphragm centered at the image point, relatively to said electrostatic deflector means. of the diaphragm of said mirror.

1' l 1 I t 

1. Means for selectively forming an ion microanalyser or a masssectrometer of the double-focusing type, said means comprising: a sliding plate having a solid first part and a second part provided with an aperture; an ion microanalyser comprising: first means for deriving secondary electrons from a sample and focusing those secondary electrons into a beam having a cross-over; first and second magnetic prisms having a common face and respective second faces; means for directing said beam toward said second face of said first magnetic prism; an electrostic reflecting means, provided with an entrance diaphragm centered at the image point of said cross-over relatIvely to said first magnetic prism, for reflecting toward said common face those of said secondary ions, let through by said diaphragm, whose energy-to-charge ratio lies under a predetermined threshold, said reflector means comprising a plurality of electrodes including a solid last electrode formed by said solid first part of said sliding plate; an ion electron converter; and means for directing the ions issuing from said second face of said second magnetic prism toward said ion converter; a mass-spectrometer comprising, in series: said first means; said first magnetic prism; said diaphragm; and energy filtering electrostatic deflector means; and an ion collector; means for imparting to said sliding plate a first position for which said first part of said plate forms said last solid electrode of said reflector means, and a second position for which said aperture of said second part is coaxial with said diaphragm; and means for switching the potentials applied to at least two of said electrodes of said reflector means according to whether said plate is in its first or second position.
 2. An apparatus as claimed in claim 1, wherein said electrostatic deflector means is a 90* spherical electrostatic deflector means, and wherein said ion collector comprises an entrance-diaphragm centered at the image point, relatively to said electrostatic deflector means, of the diaphragm of said mirror. 